Conference-XAFS Theory-Size Control of Ni Nanocluster by the Carbon Chain Length of Secondary Alkoxide

Secondary-alkoxide stabilized Ni colloid was synthesized using secondary alkoxide as a reducing and stabilizing agents. Cluster size was controlled by changing carbon chain length of secondary alkoxide; 2-octanol derived alkoxide gave the smallest nanocluster. Ni colloid thus prepared was applied to the preparation of supported Ni nanocluster catalysts. Size dependency of Ni nanocluster on catalytic activity for water gas shift reaction was observed. [DOI: 10.1380/ejssnt.2012.648]


I. INTRODUCTION
Nickel has gained much attention as catalytic materials because their nanoparticles show catalytic activity for various reactions, e.g.steam reforming [1] and hydrogenation of olefins [2].In addition, usage of Ni as catalysts is beneficial from the viewpoint of large-scale industrial application because the amount of deposit of Ni is larger than noble metals.One of the key parameters of the catalysis is the particle size [3][4][5].Higher catalytic performance is expected with reduction of the particle size because of increase in the population of low-coordination sites.However, it is difficult to reduce the particle size because of strong tendency of aggregation.Supported Ni catalysts prepared by conventional impregnation method are composed of large and polydisperse Ni particles.And hence, the development of alternative method for preparing small and monodisperse Ni nanocluster is desired.
Metal colloids have been widely used as catalysts because it is easy to control the particle size and reduce size distribution [6][7][8][9].In colloid preparation, the colloid size was affected by the nature of reducing agent and stabilizer [6,10].Caubère et al. reported that the mixture of sodium hydride / sodium alcoholates acts as a reducing agent of metal salts and predicted that the reducing power can be modified by the nature of the alcoholates [11].In this study, we attempted to control the size of the Ni colloid by changing the carbon-chain length of alkoxide.Ni colloid thus prepared was immobilized on Al 2 O 3 to suppress the aggregation during the catalytic usage.Catalytic activity of water gas shift reaction on the supported Ni nanocluster was also investigated.

A. Preparation of Ni colloid and supported Ni catalyst
NaH (4.5 mmol) was washed with hexane at several times to remove the mineral oil in a Schlenk flask under N 2 atmosphere.After removing hexane, dehydrated THF (20 mL) was added to the Schlenk flask and refluxed at 339 K.The alcohol (1.5 mmol) was added to this suspension and the mixture was stirred for 10 min at 339 K to form sodium alkoxide, and then cooled down to room temperature.In this study, five different alcohols were used as alkoxide precursors; 2-propanol (2-Pr), 2butanol (2-Bu), 2-pentanol (2-Pe), 2-octanol (2-Oc), and 2-decanol (2-De).After addition of Ni(OAc) 2 (0.5 mmol) at room temperature, this suspension was stirred under refluxed at 339K for additional 2 h to obtain black Ni(0) colloid.Excess NaH was decomposed by the addition of the alcohol (3.0 mmol) using the above.The mixture of Ni(0) colloid thus obtained and Al 2 O 3 (Aerosil, Alu-C) was stirred for 3 h, followed by solvent removal and drying in vacuo overnight.The obtained powder was washed with distilled water and dried at room temperature in vacuo overnight.The obtained catalyst was reduced by H 2 at 673 K in a closed circulating system before use for characterization and reaction.Ni loading was regulated to 3 wt%.The nomenclature of the catalysts is listed in Table I.  fitting (CF) analysis was performed by the EXAFS analysis program REX2000 (Rigaku Co.).TEM measurement was conducted to estimate the Ni cluster size by using JEM-2100F operated at 200 kV.Water gas shift reaction (WGSR) was performed at 523 K in a closed circulating system.Catalyst (0.05g) was placed in a U-shaped glass reactor connected to the closed circulating system.Initial pressures of CO, H 2 O and He were 5, 5 and 40 Torr, respectively.The CO 2 produced was detected by TCD (Shimadzu, GC-8A).

A. Characterization of Ni nanocluster catalyst
Ni K-edge XAFS measurement was conducted for the Ni nanocluster catalysts and k 3 -weighted Ni K-edge EX-AFS oscillations and FT of Ni K-edge EXAFS oscillations (k -range: 30-135 nm −1 ) are shown in Fig. 1 and Fig. 2, respectively.As shown in Fig. 1, EXAFS spectrum for every Ni nanocluster catalyst is similar to that of Ni foil, suggesting Ni(0) metal was formed by reduction with H 2 at 673 K. CF analysis was conducted for the peak positioned at 0.21 nm in Fig. 2, in k -space using Ni foil as a model compound to determine coordination number (CN ) and coordination distance (r ).CF results are listed in Table II.CF results show that CN decreases with lengthening the carbon chain length of the alcohol, to the col Ni/Al 2 O 3 2-Oc.Lengthening carbon chain longer than 2-Oc, the CN became large as 10.3 (2-De).
TEM measurement showed that every catalyst has narrow size distribution although it was difficult to precisely determine the boundaries between the particles and the background due to poor contrast arising from small particle size and low electron density of Ni.Table III shows the average particle diameter (d av ) and standard deviation (σ) estimated from the TEM images.The compari- son between the diameter and the CN also supports that 2-Oc gave the smallest nanoclusters (Table III).Hereafter, we will use the CN values as a measure of the particle size rather than the d av because the former values are more reliable.
Alkoxide plays two roles in the preparation of Ni colloid; one is reduction of Ni ions and the other is stabilization of the resulting Ni(0) clusters [12].Firstly, stronger reducing agent gives smaller clusters [8,13].On the basis of the time of color change from green to black during the preparation, we can quantitatively estimate the reducing ability as follows: 2-PrONa > 2-BuONa > 2-PeONa > 2-OcONa > 2-DeONa.Secondary, it is expected that the alkoxide with longer carbon chain length can stabilize the clusters more against aggregation because of stronger van der Waals attractive force between the carbon chains.On the basis of the above consideration, the formation of smallest clusters in 2-OcONa is ascribed to the optimization http://www.sssj.org/ejssnt(J-Stage: http://www.jstage.jst.go.jp/browse/ejssnt/)    between these two competing effects.Figure 3 shows the relationship between CN and the initial rate of water gas shift reaction.Contrary to a simple expectation, the catalytic activity did not increase with decrease in size, but showed a maximum at a certain size.Similar volcano-like behavior was observed in the aerobic oxidation of cyclohexane catalyzed by Au clusters supported on hydroxyapatite [14].We believe that other effects, such as morphology of Ni nanocluster, electronic effect by the remaining ligand, play a role.The further investigation will be performed.

IV. CONCLUSION
The size of Ni nanocluster was controlled by changing the carbon chain length of secondary alkoxide.2-OcONa (2-octanol derived alkoxide) gave the smallest nanocluster as a result of the optimization between the conflicting dependency of reducing and stabilizing ability on the carbon chain length.Ni nanocluster catalysts showed the activity for water gas shift reaction.It was revealed that there is an optimal size of Ni nanocluster for the catalysis.

B.
Characterization and catalytic reactionNi K-edge XAFS measurements were conducted at AR NW10A of PF (Si(311) double crystal, 2010G064) and at BM-23 of ESRF (Si(111) double crystal, CH3497, Priority Program for Disaster-Affected Quantum Beam Facilities) in a transmission mode at room temperature.Curve-e-Journal of Surface Science and NanotechnologyVolume 10 (2012)

FIG. 3 :
FIG.3: Relationship between CN and initial rate of water gas shift reaction.

TABLE I :
Nomenclature of catalysts and used alcohol for colloid preparation.
a Coordination number for Ni-Ni.b Coordination distance.

TABLE III :
Average particle size estimated by TEM and coordination number of Ni-Ni.Average particle size of Ni nanocluster estimated by TEM.b Standard deviation of the Ni nanocluster size distribution.
a c Coordination number for Ni-Ni.

TABLE IV :
Initial rate of water gas shift reaction at 523 K.
a Initial rate of CO 2 production.

Water gas shift reaction WGSR
(CO+H 2 O−→CO 2 +H 2 ) was carried out in a closed circulating system at 523 K on the Ni nanocluster catalysts.The initial rates (r 0 ) based on the CO 2 production are listed TableIV.The highest activity was achieved on col Ni/Al 2 O 3 2-Pr, whereas col Ni/Al 2 O 3 2-De showed the lowest catalytic activity.Deviations of the initial rates obtained for the catalysts prepared in different batches are typically less than 5%.Table IV indicates that the catalytic activity for WGSR depends on the Ni cluster size. B.